The present invention relates generally to communication networks, and in particular, to Push-To-Talk-over-Cellular (PoC) services in wireless communication networks.
Push-To-Talk-over-Cellular (PoC) is a fast-connect, half-duplex service alternatively known as Push-to-Talk (PTT) or voice chat services. PTT applications provide two-way, one-to-one private calls and one-to-many group call services similar to a walkie-talkie service, but with a number of advantages. Subscribers can take advantage of private, direct, simultaneous radio voice connection over very wide areas, affordably and conveniently, often replacing other devices such as pagers and two-way radios. Subscribers can instantly connect to one or more receivers simply by pushing a button on their handset. Users receiving the call hear the caller's voice automatically without having to answer the call.
PoC is the official name used in the open specifications currently being developed by the Open Mobile Alliance (OMA) for this service. The PoC specifications utilize a number of existing specifications from the Internet Engineering Task Force (IETF), the Third Generation Partnership Project (3GPP), and 3GPP2 including the capabilities of the 3GPP IP Multimedia Subsystem (IMS) and the 3GPP2 Multimedia Domain (MMD) to enable IP connectivity between mobile devices. Technical specifications describing the PoC feature include the 3GPP TS 22.141 and TS 23.141. Additionally, 3GPP 24.841 specifies the presence methods using the Session Initiation Protocol (SIP). All of these specifications are hereby incorporated herein by reference in their entireties.
In one solution, PoC uses a server, which is independent of the existing radio and core networks. PoC may be implemented on the IP backbone over radio technologies such as WCDMA, CDMA2000, GPRS, UMTS, and 802.11.
The use of IMS and MMD adds the ability to integrate voice and data services over IP-based packet-switched networks. Two fundamental capabilities are added on top of the packet-switched domain. First is the ability to find a user via the Session Initiation Protocol (SIP) to initiate a session. Second is the ability to integrate new services such as PTT. By introducing IMS and its multiple service-delivery capability, a common infrastructure for services is established, and the requirement to build a service delivery infrastructure for each service is eliminated.
A current weakness of the PoC service is that call setup time takes too long, compared to walkie-talkie services. With PoC, the total call setup delay results from several smaller delays, including the necessity (generally) of paging the called MSs prior to forwarding the SIP INVITE message to the MSs. Paging can take up to 1.5 seconds, which, in Global System for Mobile Communications (GSM)/General Packet Radio Service (GPRS) networks, represents approximately ⅓ of the total initial call setup delay. This delay is mainly significant when auto-answer mode is used.
Paging is required because (for GSM/GPRS) the packet-idle MSs drop the radio connections and enter packet mobility management state STANDBY after approximately 45 seconds of inactivity on the packet session. Paging is necessary to reestablish the READY state, and this adds approximately 1.5 seconds to the call setup time. Paging is not necessary if the called MS is already in READY state.
Currently, the standardized method for call setup is SIP signaling over the primary PDP context. This does not reduce the time required for paging the terminal that is in STANDBY mode, so the problem is currently not solved. It has been variously proposed to use “keep-alive” methods, but these methods involve periodically sending “heartbeat” signals to maintain the READY state, but this adds a prohibitive amount of signaling, ties up radio resources, and causes unnecessary power consumption in the MS.
In existing call setup, each MS having a PoC client initially registers with a PoC server using SIP signaling. Next, the MS registers with a presence server utilizing a SIP PUBLISH message to indicate that the MS is available. Periodically the MS re-registers with the presence server (also utilizing the SIP PUBLISH message) to indicate that the MS is still available for PoC services. The MS updates the presence information whenever there is a change. The MS also de-registers at DETACH or power-off.
With some implementations of PoC, the calling user first selects a ‘contact’ list from the menu on his MS. This list contains the presence/availability status of each listed party. The PoC client in the calling user's MS requests that the presence status be refreshed at the time that the ‘contact’ list is selected. The PoC client sends a SUBSCRIBE request to the presence server (via the PoC server), and the presence server sends back the stored status of the registered contacts (or group members). When the MS subscribes to a contact list (instead of an individual MS), the SUBSCRIBE request goes first to the Resource List Server (RLS). The RLS propagates the individual SUBSCRIBE requests to the presence server.
This information is used to refresh the ‘contact’ list presence information. Alternatively, the presence server may automatically push this information to the PoC client whenever a change of status is detected, but this causes more signaling while the updates are only needed when the subscriber is viewing the contact list.